1. Field of the Invention
It is necessary to always properly control the air-to-fuel ratio of a gas mixture supplied to an engine when considering the problems of countermeasure against engine exhaust gas and fuel consumption. The present invention relates to an engine air-to-fuel ratio control system which satisfactorily meets such requirements.
2. Description of the Prior Art
In the past, a gas mixture supplied to a combustion chamber of an internal combustion engine has been maintained richer than gas mixture having an optimum air-to-fuel ratio (A/F) which assures perfect combustion, in order to assure positive ignition of the gas mixture. Such gas mixture exhibited imperfect combustion resulting in noxious gases such as carbon monoxide (CO) and hydrocarbon (HC), which has recently raised serious social problem. Further, since a relatively rich gas mixture is supplied, fuel consumption is considerable. To overcome the above problems, many approaches have been developed to detect the air-to-fuel ratio by an air-to-fuel ratio detector using a metal oxide semiconductor based on zirconium dioxide (ZrO.sub.2) to adjust the gas mixture to have an optimum air-to-fuel ratio.
In this type of air-to-fuel ratio control system, a carburetor is so arranged as to apply air-fuel mixture to an engine, whose air-to-fuel ratio is slightly lower than an optimum ratio, and the air-to-fuel ratio of the mixture supplied to the engine is controlled by auxiliary air being admitted through an auxiliary passage provided in the carburetor in addition to a main passage.
The amount of air supplied through the auxiliary passage is controlled by a bypass valve provided in the auxiliary passage, which is driven by a control circuit responding to an output signal of an air-to-fuel ratio detector mounted in an exhaust pipe of the engine. However, in the above system, because of the system delay, that is, a time delay from the supply of the gas mixture to the detection of the air-to-fuel ratio by the air-to-fuel ratio detector, a problem has occurred in that as the rate of opening of the control valve is increased the air-to-fuel ratio rapidly converges to the required air-to-fuel ratio during high load, high speed operation or during transient state but the control range thereof is widened particularly during low load and low speed operation.